Level 45 is a real tricky one, actually, I'm stuck on it too. Of all the places I've been, here are the best answers of how to solve it.
1. Don't stop running when you are going through the course....
Don't shoot all of those brown blocks. Just shoot the one on the left or middle (left is best)! Don't shoot too many at once. Once you have shot one pop some of it's smaller bubbles before you shoot...
Well there are is only one cheat on this level and its on top of the bouncy thingy but it only gives you ice which doesnt really help you, so my advice is to stay at the bottom at just try to shoot...
look it up on youtube.com
It is called how to beat level 72 in bubble struggle 2 rebubbled walkthrough and it will walk you right through the entire level.